1. Technical Field
The present inventive concepts relate to a photolithography technique and, more particularly, to methods of forming a photo mask.
2. Description of Related Art
A photolithography technique used in fabrication of a semiconductor device may generally employ a method of transferring pattern images of a photo mask onto a wafer through optical lenses of an optical system. However, as semiconductor devices become more highly integrated, the sizes of the mask patterns are becoming increasingly closer to a wavelength of a light emitted from a light source of the optical system. As a result, the photolithography technique may be increasingly influenced by diffraction and/or interference of the light of the optical system. In particular, the optical system projecting the pattern images may act as a low-pass filter. Thus, photoresist patterns formed on the wafer may exhibit poor profiles which are distorted relative to the original mask patterns.
In more detail, if the size (or period) of the mask patterns formed on the photo mask increases, a spatial frequency of the mask patterns may decrease. In this case, the photo mask may transmit a light having a relatively high order of frequency. Thus, the photoresist patterns formed on the wafer may exhibit a profile that is similar to the original mask patterns. However, a high frequency area such as a corner region may still be formed to have a distorted image. This image distortion effect is referred to as an optical proximity effect (OPE). The more the size of the mask patterns is reduced, the higher the spatial frequency of the mask patterns becomes. Thus, if the size of the mask patterns is reduced, severe image distortion can occur due to the optical proximity effect.